Crustal evolution of the Pitangui Synclinorium region, southern São Francisco Craton (Brazil): insights from zircon U-Pb and Lu-Hf isotopes of Rhyacian metasedimentary and metavolcanic rocks

In the southern São Francisco Craton (Brazil), the Fazenda Tapera Formation comprises siliciclastic metasedimentary rocks outcropping in the central and northwestern regions of the Pitangui Synclinorium. This unit records an association of arkosic metasandstones, metagreywackes, metarhythmites and subordinate metatufts, and outcrops as a synform with NW-SE-trend. It is overlapped by Neoproterozoic sedimentary rocks and surrounded by an Archean greenstone belt and TTG complexes. This formation comprises two members named Engenho Velho and Fazenda Pacheco, showing transitional contact among them. The former mainly comprises metarhythmites, with subordinate meta-arkose, metagreywacke, felsic metatuffs (with and without accretionary lapilli) and metachert levels. The top member is composed of lithic meta-arenite, meta-arkose and metagreywacke, locally interbedded with metapelites and metatuffs, which decrease towards the top. U-Pb and Lu-Hf LASS-ICP-MS analyses were obtained on zircon grains of a felsic metatuff from the basal member. These zircon grains reveals a maximum age at ca. 2134 ± 27 Ma for the basal volcanic ash/fall deposit, and chondritic εHf(t) values from +2.8 to +7.9, suggesting juvenile magma source. In addition, a LA-ICP-MS U-Pb zircon maximum depositional age of 2137 ±18 Ma were obtained from a quartzite also from the Engenho Velho basal member. Besides a Rhyacian cluster between 2120-2263 Ma, the age distribution pattern of this sample shows a secondary cluster between 2667-2937 Ma, with peak at ca. 2806 Ma. Additionally, three grains have ages at ca. 3119 Ma, 3199 Ma and 3304 Ma. The Rhyacian detrital zircons yielded subchondritic to chondritic  εHf_(t) values (-18.0 to +7.2), indicating both juvenile and crustal sources, and possibly different degrees of mixing between ancient crust and juvenile magmas. Neo- and Mesoarchean (2667-2884 Ma) εHf_(t) values on zircons show subchondritic data between -11.9 to -1.2, suggesting at least a crustal component. An older 2988 Ma zircon reveals a chondritic  εHf_(t) value of  +3.1. At the top of the unit (Fazenda Pacheco member), a metagreywacke has a LA-ICP-MS U-Pb zircon maximum depositional age of 2132 ±13 Ma. This sample reveals an age spectra showing four peaks at ca. 2153 Ma, 2717 Ma, 2870 Ma and 2947 Ma. In addition, three older zircon grains have ages at ca. 3096 Ma, 3159 Ma and 3365 Ma. The Hf isotope compositions (LA-ICP-MS) of Rhyacian zircons from this metagreywacke are very subchondritic to chondritic, with a wide range of εHf_(t) values between -25.6 to 5.2, suggesting a period of juvenile magma production, as well as crustal recycling and mixing of material derived from both juvenile and crustal sources. The Neoarchean zircon grains, with ages between 2503 Ma and 2722 Ma, show εHf_(t) in the range of -7.8 to - 0.5, indicating crustal reworking. The Mesoarchean zircons (2845 Ma to 3159 Ma) have εHf_(t) values between -8.7 to +1.3, suggesting a mixture of juvenile and reworked crust. The above data advance understanding of crustal evolution of the Pitangui Synclinorium region. The source rocks for the Fazenda Tapera Formation are associated with the main magmatic events recorded in the Southern São Francisco Craton. A tectonic relationship between the youngest dates and the ultimate magmatic arc of the Mineiro Belt (Minas Accretionary Orogeny) is apparent, as also suggested by the Hf isotopic constraints. The wide εHf_(t) variations of the Rhyacian zircon grains point to magmatic arc environments, with mixing between ancient crust and juvenile magmas. The contrast between the Hf signatures of the Mesoarchean and Neoarchean sources indicates that, although there was crustal reworking in the Mesoarchean, it was intensified in the Neoarchean, with a long period of crustal recycling.